The immunoblot analyses of convalescent sera from groups of chimpanzees that were either experimentally-infected, immunized and challenged or rechallenged following a previous infection were examined and compared in an attempt to identify the protective immunogens. The sera of the infected chimpanzee reacted with 17 to 20 protein bands and the immunoblot patterns produced were remarkably similar to the convalescent serum of a patient with natural-occurring Mycoplasma pneumoniae disease. Whereas the sera of chimpanzees immunized with the acellular vaccine recognized three proteins (61, 42 and 30 kDa), the sera of the formalin-inactivated vaccine were sero-negative prior to challenge. All immunized animals seroconverted after challenge. Although the immunoblot patterns among the three groups were similar following challenge, a moderate degree of variations was noted among individuals and between groups. The previously infected chimpanzees, who were fully protected, showed the most impressive immunologic responses on challenge. The most immunodominant component on challenge in each group was the 169 kDa protein band, that co-migrated with the well-established P1-adhesin. The other immunodominant protein bands were the 117, 86, 35 and 30 kDa. Of these, the 86 and 35 kDa polypeptides were most noteworthy because these two surface exposed, immunodominant proteins were present in the cytadsorbing, pathogenic strains PI-1428, M129 and FH, but were absent in the non-cytadsorbing, non-pathogenic strain B176. These components were also the most immunodominant components in the convalescent sera of patients with naturally-occurring disease. For these reasons, the 169, 86 and 35 kDa proteins probably represent the most promising immunogens for inclusion in the development of a protective acellular vaccine.